It is often desirable to separate one or more desired compounds from a liquid mixture containing the desired compounds and other components. The other components may be similar in chemical nature to the desired compounds which makes separation by conventional methods difficult, energy intensive or financially costly. For example, the separation of a light hydrocarbon compound, such as hexane from a liquid mixture of heavy hydrocarbons in a petroleum refining process can require complex and high energy consuming distillation. In other cases, the desired compound can be a component of a mixture that forms an azeotrope with the other components. In that case, many conventional separating techniques to obtain the desired compound can be futile.
When a suitable combination of desired compound, liquid mixture and selectively permeable membrane can be identified, the desired compound can be removed from the liquid mixture by selectively permeating the mixture through the membrane. The desired compound or compounds can permeate either faster or slower than other mixture components to achieve the intended separation. Of interest is a membrane separation process known as organic solvent nanofiltration or solvent resistant nanofiltration (hereinafter collectively referred to as “nanofiltration”). See for example, Volkov, et al., High permeable PTMSP/PAN composite membranes for solvent nanofiltration, Journal of Membrane Science 333 (2009) pp. 88-93. Characteristic features of nanofiltration are that the feed and permeate fluids in contact with the membrane are present in the liquid state and that the driving force for permeation is hydraulic pressure gradient from feed to permeate sides.
In membrane separations the membrane composition should be substantially unaffected by the liquids being separated. This can be problematic for nanofiltration processes applied to separating liquid mixtures of or containing organic solvents such as hydrocarbons. Such solvents can react with and/or solubilize the membrane under preferred conventional nanofiltration conditions. Hence it is desired to have a composition that can be formed into a membrane, that is selectively permeable to the liquid components being separated and that is not affected by contact with those components.
Composite membranes are commonly used for nanofiltration. One of the traditional methods of making composite membranes calls for dissolving a polymeric composition for the membrane in a suitable solvent and casting the resulting solution on a support. The support wet with casting solution normally is thoroughly dried to remove substantially all of the solvent. Typically solvent is removed by heating the wet membrane to a high temperature to speed up volatilization of the solvent. Drying temperatures are limited by the phase transition temperature of the polymeric compositions. If the temperature is too high, the membrane can soften, deform and even break. Separation membranes are vulnerable to such distortion because they are usually made to extremely small thicknesses to improve transmembrane flux.
It is desirable to have a membrane for a nanofiltration process that provides a high flux of the permeating liquid component It is also desirable that the membrane is inert to a wide variety of organic solvents so that it can be used to separate mixtures of organic liquids.